1. Field of the Invention
This invention relates to medical implants formed of a polymeric material such as ultra-high molecular weight polyethylene, with superior oxidation resistance upon irradiation and a method for making the same.
2. Description of the Prior Art
Various polymer systems have been used for the preparation of artificial prostheses for biomedical use, particularly orthopedic applications. Among them, ultra-high molecular weight polyethylene is widely used for articulation surfaces in artificial knee and hip replacements. Ultra-high molecular weight polyethylene (UHMWPE) has been defined as those linear polyethylenes which have a relative viscosity of 2.3 or greater at a solution concentration of 0.05% at 135.degree. C. in decahydronaphthalene. The nominal weight--average molecular weight is at least 400,000 and up to 10,000,000 and usually from three to six million. The manufacturing process begins with the polymer being supplied as fine powder which is consolidated into various forms, such as rods and slabs, using ram extrusion or compression molding. Afterwards, the consolidated rods or slabs are machined into the final shape of the orthopedic implant components. Alternatively, the component can be produced by compression molding of the UHMWPE resin powder.
All components must then go through a sterilization procedure prior to use, but usually after being packaged. There exists several sterilization methods which can be utilized for medical applications, such as the use of ethylene oxide, heat, or radiation. However, applying heat to a packaged polymeric medical product can destroy either the integrity of the packaging material (particularly the seal, which prevents bacteria from going into the package after the sterilization step) or the product itself.
Because ethylene oxide may adversely impact environmental and employee safety, gamma ray, x-ray or electron beam radiation has been utilized as a preferred means of sterilization. These types of radiation use a high energy beam to kill bacteria, viruses, or other microbial species contained in the packaged medical products, achieving the goal of product sterility.
However, it has been recognized that regardless of the radiation type, the high energy beam causes generation of free radicals in polymers during radiation. It has also been recognized that the amount of free radicals generated is dependent upon the radiation dose received by the polymers and that the distribution of free radicals in the polymeric implant depends upon the geometry of the component, the type of polymer, the dose rate, and the type of radiation beam. The generation of free radicals can be described by the following reaction (which uses polyolefin and gamma ray irradiation for illustration): ##STR1##
Depending whether or not oxygen is present, primary free radicals r.cndot. will react with oxygen and the polymer according to the following reactions as described in "Radiation Effects on Polymers", edited by Roger L. Clough and Shalaby W. Shalaby, published by American Chemical Society, Washington, D.C., 1991.
In the Presence of Oxygen ##STR2## EQU rO.sub.2 .cndot.+polyolefin.fwdarw.rOOH+P.cndot. (3) EQU P.cndot.+O.sub.2 .fwdarw.PO.sub.2 .cndot. (4) ##STR3## EQU rO.sub.2 .cndot., PO.sub.2 .cndot..fwdarw.Some chain scission products(6) ##STR4## EQU P.cndot.+PO.sub.2 .cndot..fwdarw.POOP (ester cross-links) (8) EQU 2P.cndot..fwdarw. P--P (--C-C cross-links) (9)
In radiation in air, primary free radicals r.cndot. will react with oxygen to form peroxyl free radicals rO.sub.2 .cndot., which then react with polyolefin (such as UHMWPE) to start the oxidative chain scission reactions (reactions 2 through 6). Through these reactions, material properties of the plastic, such as molecular weight, tensile, and wear properties, are degraded.
Recently, it was found that the hydroperoxides (rOOH and POOH) formed in reactions 3 and 5 will slowly break down as shown in reaction 7 to initiate post-radiation degradation. Reactions 8 and 9 represent termination steps of free radicals to form ester or carbon-carbon cross-links. Depending on the type of polymer, the extent of reaction 8 and 9 in relation to reactions 2 through 7 may vary. For irradiated UHMWPE, a value of 0.3 for the ratio of chain scission to cross-linking has been obtained, indicating that even though cross-linking is a dominant mechanism, a significant amount of chain scission occurs in irradiated polyethylene.
By applying radiation in an inert atmosphere, since there is no oxidant present, the primary free radicals r.cndot. or secondary free radicals P.cndot. can only react with other neighboring free radicals to form carbon-carbon cross-links, according to reactions 10 through 12 below. If all the free radicals react through reactions 10 through 12, there will be no chain scission and there will be no molecular weight degradation. Furthermore, the extent of cross-linking is increased over the original polymer prior to irradiation. On the other hand, if not all the free radicals formed are combined through reactions 10, 11 and 12, then some free radicals will remain in the plastic component.
In an Inert Atmosphere EQU r.cndot.+polyolefin.fwdarw.P.cndot. (10) EQU 2r.cndot..fwdarw.r--r (C--C cross-linking) (11) EQU 2P.cndot..fwdarw.P--P (C--C cross-linking) (12)
It is recognized that the fewer the free radicals, the better the polymer retains its physical properties over time. The greater the number of free radicals, the greater the degree of molecular weight and polymer property degradation will occur. Applicant has discovered that the extent of completion of free radical cross-linking reactions is dependent on the reaction rates and the time period given for reaction to occur.
Several prior art patents attempt to provide methods which enhance UHMWPE physical properties. European Patent Application 0 177 522 B1 discloses UHMWPE powders being heated and compressed into a homogeneously melted crystallized morphology with no grain memory of the UHMWPE powder particles and with enhanced modulus and strength. U.S. Pat. No. 5,037,928 discloses a prescribed heating and cooling process for preparing a UHMWPE exhibiting a combination of properties including a creep resistance of less than 1% (under exposure to a temperature of 23.degree. C. and a relative humidity of 50% for 24 hours under a compression of 1000 psi) without sacrificing tensile and flexural properties. U.K. Patent Application GB 2 180 815 A discloses a packaging method where a medical device which is sealed in a sterile bag, after radiation/sterilization, is hermetically sealed in a wrapping member of oxygen-impermeable material together with a deoxidizing agent for prevention of post-irradiation oxidation.
U.S. Pat. No. 5,153,039 relates to a high density polyethylene article with oxygen barrier properties. U.S. Pat. No. 5,160,464 relates to a vacuum polymer irradiation process.